Microcircuit fabrication requires that precisely controlled quantities of impurities be introduced into tiny regions of the silicon substrate. Subsequently, these regions must be interconnected to create components and VLSI circuits. The patterns that define such regions are created by a photolithographic process.
As semiconductor devices become more highly integrated, the line width of VLSI circuits typically becomes scaled down. The semiconductor industry's drive toward integrated circuits with ever decreasing geometries, coupled with its pervasive use of highly reflective materials, such as polysilicon, aluminum, and metal suicides, has led to increased photolithographic patterning problems. Unexpected reflections from these underlying materials, during the photoresist patterning step, result in the photoresist pattern being distorted.
This problem is further compounded when the photolithographic process is in the ultraviolet (UV) or deep ultraviolet (DUV) wavelength range. The patterns formed in the photoresist are easily compromised by the effects of uncontrolled reflections from the underlying materials due to the increased optical metallic nature of the underlying reflective materials at these wavelengths. Therefore, the fabrication of advanced integrated circuits with submicron geometries is limited.